The present invention is related to the field of cellular telecommunications. More particularly, the present invention involves adjusting the tilt angle of a base station antenna in a target cell for the purpose of improving the uplink signal quality received by the base station and for improving the signal quality received by mobile units operating in co-channel cells.
In a cellular telecommunications system (e.g., a cellular mobile telecommunications system) maintaining and/or improving speech quality is of great importance. One factor which can significantly and adversely affect speech quality is the presence of co-channel interference. Co-channel interference occurs when two or more cells (i.e., co-channel cells) located adjacent to one another or in relatively close proximity to one another reuse (i.e., share) the same frequency or set of frequencies. In essence, a signal being transmitted over a reused frequency in one cell is perceived as interference in the other cell.
One way in which co-channel interference can be avoided is to assign a group of dedicated frequencies to each cell in the system so that no two cells reuse the same frequency. Unfortunately, there are but a limited number of frequencies available to cover an ever increasing demand for cellular service. Accordingly, assigning a dedicated group of frequencies to each cell is generally not a feasible solution to the co-channel interference problem.
Another technique which is often used to minimize, rather than eliminate, co-channel interference involves maximizing reuse distance. Reuse distance is generally understood to be the distance between two cells (i.e., co-channel cells) that reuse the same frequency or set of frequencies. As one skilled in the art will readily understand, as reuse distance increases, co-channel interference decreases (i.e., signal strength diminishes with distance). However, as the demand for cellular service increases, while the number of available frequencies remains the same, cellular service providers are forced to establish additional cells, which generally have smaller coverage areas. This, in turn, tends to decrease rather than increase reuse distance. Consequently, techniques that rely on increasing reuse distance to counter the effect of co-channel interference are also not an overly practical solution.
Yet another method for reducing co-channel interference involves adjusting the orientation or tilt angle of the base station antenna. In general, the base station antenna transmits and receives telecommunications signals to and from the various mobile units operating within the corresponding cell, herein referred to as the target cell. By repositioning the antenna so that the antenna beam points further and further below the horizon, the energy associated with the antenna beam is, to a greater extent, directed into the target cell and away from any adjacent cells or co-channel cells in close proximity to the target cell. Consequently, uplink interference received by the base station antenna in the target cell is reduced, as is co-channel interference received by mobile units operating in co-channel cells caused by transmissions emanating from the base station antenna in the target cell.
As with the other above-identified techniques for avoiding or minimizing co-channel interference, repositioning the base station antenna to reduce co-channel interference is not without trade-offs. The primary tradeoff associated with repositioning the base station antenna is best illustrated by FIG. 1. In FIG. 1, if the tilt angle 101 is increased, thereby causing the peak of the antenna beam 103 to be directed inward away from the target cell's boundary 105, the signal strength or carrier-to-interference ratio (i.e., C/I) will undesirably decrease for those signals being transmitted between the base station and mobile units operating in the target cell at or near the target cell boundary 105. Stated differently, an increase in the tilt angle 101 of the antenna beam 103 effectively reduces the coverage area of the target cell despite the fact that it also reduces the level of co-channel interference in the target cell as well as in nearby co-channel cells. Accordingly, it is important to determine the antenna tilt angle at which co-channel interference has been sufficiently reduced, while minimizing the loss in coverage area associated with the target cell.
Despite the fact that both interference reduction and target cell coverage area reduction should be taken into consideration when determining an optimum antenna tilt angle, there are no known cellular telecommunication systems which employ such a technique. For example, U.S. Pat. No. 4,249,181 ("Lee") describes a system wherein the level of co-channel interference in a co-channel cell is reduced by tilting the antenna beam downward by a predetermined amount. More specifically, the antenna beam is redirected such that the "notch" in the antenna pattern between the main lobe and the first side lobe is generally pointing in the direction of the neighboring or co-channel cell. While this technique reduces co-channel interference in the target and co-channel cells, there is no guarantee that the signal quality for all mobile units in the target cell as a whole has been improved because in redirecting the antenna beam, the coverage area associated with the target cell may have been substantially reduced, effectively leaving mobile units operating at the boundary of the target cell without service.
In another example, U.S. Pat. No. 5,093,923 ("Leslie") describes adjusting antenna tilt angle to reduce interference. More particularly, Leslie is concerned with adjusting the orientation of an antenna associated with a cellular repeater or booster relative to the donor cell's antenna. However, the orientation of the repeater or booster antenna that is used for transmitting and receiving signals with mobile units does not change. Therefore, the region covered by the repeater and the boundary of the donor cell (i.e., target cell) are unaffected by the orientation of the antenna. Accordingly, Leslie does not take into consideration target cell coverage area in determining the most appropriate antenna tilt angle.
As explained, neither of the two existing designs for reorienting antenna tilt angle, nor any other known designs, take target cell coverage reduction into consideration. Nevertheless, target cell coverage reduction is an important consideration in determining optimal antenna tilt angle. Accordingly, it would highly desirable to provide a technique to reduce co-channel interference by optimizing antenna tilt angle that takes into consideration both co-channel interference reduction as well as target cell coverage area reduction.